1. Field of the Invention
The present invention relates generally to power supplies and, more specifically, the present invention relates to a switched mode power supply controller.
2. Background Information
Electronic devices use power to operate. Switched mode power supplies are commonly used due to their high efficiency and good output regulation to power many of today's electronic devices. In a known switched mode power supply, a low frequency (e.g. 50 Hz or 60 Hz mains frequency), high voltage alternating current (AC) is converted to high voltage direct current (DC) with a diode rectifier and capacitor. The high voltage DC is then converted to high frequency (e.g. 30 to 300 kHz) AC, using a switched mode power supply control circuit. This high frequency, high voltage AC is applied to a transformer to transform the voltage, usually to a lower voltage, and to provide safety isolation. The output of the transformer is rectified to provide a regulated DC output, which may be used to power an electronic device. The switched mode power supply control circuit provides usually output regulation by sensing the output controlling it in a closed loop.
A switched mode power supply may include an integrated circuit power supply controller coupled in series with a primary winding of the transformer. Energy is transferred to a secondary winding from the primary winding in a manner controlled by the power supply controller to provide the clean and steady source of power at the DC output. The transformer of a switched mode power supply may also include another winding called a bias or feedback winding. The bias winding provides the operating power for the power supply controller and in some cases, it also provides a feedback or control signal to the power supply controller. In some switched mode power supplies, the feedback or control signal can come through an opto-coupler from a sense circuit coupled to the DC output. The feedback or control signal may be used to modulate a duty cycle of a switching waveform generated by the power supply controller or may be used to disable some of the cycles of the switching waveform generated by the power supply controller to control the DC output voltage.
In a power supply controller, where the bias supply power and feedback current are combined to a single electrical terminal, an output overshoot can occur on the power supply output during power-up. To illustrate, FIG. 1 is a diagram showing a power supply 101 including a power supply controller 139 on which bias supply power and feedback current are combined to a single electrical terminal 145. A bridge rectifier 105 and capacitor 107 are coupled to rectify and filter an input alternating current (AC) voltage received at AC mains 103. The rectified voltage is received at a primary winding 115 of a transformer 113. The transformer 113 includes a secondary winding 117 and a bias winding 119. Diode 121 and capacitor 123 rectify and filter the secondary output, whereas diode 125 and capacitor 127 rectify and filter the bias winding. The power supply controller includes a drain terminal 141 coupled to the primary winding 115 and a source terminal 143 coupled to ground. The power supply controller 139 switches a power switch 147 at a predetermined frequency to transfer the energy to the secondary winding 117 using transformer 113. Diode 109 and Zener 111 are used for clamping the drain.
A feedback loop is formed from DC output 129 through output sense circuit 131 to a control terminal 145 of the power supply controller 139. The feedback loop includes the output sense circuit 131 and opto-coupler 133. The opto-coupler 133 includes a transistor 135 that is optically coupled to a photodiode 137. The combined bias supply current as well as the feedback current is provided to the control terminal 145 by the opto-coupler 133 using the bias winding 119. Thus, the control terminal 145 may be characterized as a supply voltage (V.sub.S)/feedback terminal for power supply controller 139. This control terminal 145 is therefore frequently referred to as a combined electrical terminal. An external capacitor 151 is connected between control terminal 145 and ground.
Power supply controller 139 is a type of controller that typically uses a shunt regulator and extracts the feedback signal from the shunt current. The extracted feedback signal is derived from the excess current to the bias supply current. Power supply controller 139 usually contains a current source from the drain terminal 141 of the power switch 147 to charge the control terminal 145 to a control terminal regulation voltage during power-up before the power switch 147 can switch. This is illustrated in FIG. 2 with control terminal 145 shown being charged to control terminal regulation voltage 207 at time 201.
Once the switching starts, which is shown in FIG. 2 with drain terminal 141 being switched after time 201, the current source from the drain terminal 141 is typically turned-off. At this time, the bias supply current requirements of power supply controller 139 are typically provided by the external capacitor 151 that is connected to this combined electrical control terminal 145. The external capacitor 151 provides the bias supply requirements until the output reaches regulation. This is illustrated in FIG. 2 between time 201 and 203 with control terminal 145 being discharged from control terminal regulation voltage 207 to voltage 209. When the power supply output reaches the output regulation voltage, which is shown in FIG. 2 as DC output 129 reaching output regulation voltage 213, the external feedback circuitry that includes output sense circuit 131 and opto-coupler 133 provides current to combined electrical control terminal 145. The current provided to control terminal 145 is the sum of the bias supply current of power supply controller 139 and the feedback current. The duty cycle of the power supply controller 139 is inversely proportional to the feedback current to regulate the output voltage.
The problem with this approach is that during power-up of DC output 129, the external capacitor 151 connected to control terminal discharges to provide the bias supply current to power supply controller. This discharge is illustrated in FIG. 2 with control terminal 145 being discharged from control terminal regulation voltage 207 to voltage 209 between time 201 and time 203. Therefore, when DC output 129 reaches its output regulation voltage 213 and the current flows through the opto-coupler 133, the external capacitor 151 connected to the combined electrical control terminal 145 first needs to charge back-up to the control terminal regulation voltage 207. During this time, no current flows into the shunt regulator of power supply controller 139. Hence there is no feedback current, and the duty cycle remains at maximum level, even though the DC output voltage is at or above the output regulation voltage. This results in an output overshoot at the power supply output. This output overshoot is illustrated in FIG. 2 with DC output rising above the output regulation voltage 213 to voltage 211 at time 205. Note that the voltage at control terminal 145 is charged back up to the control terminal regulation voltage between time 203 and time 205.
This problem is exacerbated when a soft start circuitry is employed in power supply controller 139. The soft-start circuitry typically increases the time for the DC output 129 to reach the output regulation voltage, resulting in the external capacitor 151 discharging to an even lower voltage. It then takes longer for the capacitor 151 to charge back-up and this may result in even larger output over-shoot that the soft-start circuit was intended to reduce.